Will we ever have a universal anaesthetic breathing system?

Robert J. Byrick FRCPC 0 D. Keith Rose FRCPC 0 0 Department of Anaesthesia, St. Michael's Hospital , 3~3Bond Street, Toronto, Ontario, MSB IW8 W i l l w e e v e r h a v e a u n i v e r s a l a n a e s t h e t i c breathing s y s t e m ? - breathing and hence alveolar CO2 tensions were not evaluated. Such factors as mask anaesthesia with an increase in deadspace, 5 respiratory waveform changes with various anaesthetic agents, 6 and the effect of altering the resistance of relief valves within the breathing system have not been considered. The universal application of a single breathing system requires that this technique be equally safe for all patients when used with a standardized FGF rate. What are safe levels of carbon dioxide found in inspired gases and what is the best measurement technique for determining inspired PCO2? Humphrey et a l . ' s conclusions are based on midstream samples measured at the mouth. Some authors 6 have chosen to measure inspired CO2 sampled at the carina to minimize sampling error and assess the volume of the CO2 inspired beyond the upper airway. Humphrey7 himself recognizes that "determination of the point at which rebreathing becomes clinically significant will continue to present a problem." He has chosen to use the point at which the minimum inspired CO2 tension reached 2 nmal-Ig (0.3 per cent). Conway has objected to this definition of rebreathing, s Humphrey's end-point means that some CO2 is inhaled into the upper airway throughout the inspiratory phase and may participate in gas exchange. Just how much CO2 reaches the alveoli was not determined in this study. Is this degree of rebreathing always associated with elevated PaCO2 levels without other factors being altered? Before one can generalize that no rebreathing occurs with the A_.D.E. circuit in the "A mode" at a FGF of 50 ml'kg- ~-min- ~, more investigation is needed. The most efficient use of fresh gas with any circuit during spontaneous ventilation occurs when flows are set such that rebreathing is just detected. If this flow equals the patient's alveolar ventilation then all fresh gas participates in alveolar gas exchange, none is wasted, and fractional utilization is unity (100 per cent). In the current study, when rebreathing was detected, Humphrey et al, l calculated the A.D.E. utilized 73 percent of the fresh gas (51.4---5.2ml'kg-l-min -l FGF) compared to 51 per cent for the Magill circuit (71.2 --- 6.0 ml.kg- ~. min-I FGF). By adding tubing for an expiratory limb and moving the expiratory valve downstream the functional characteristics and efficiency of Mapleson's 3 original A system appear to have been improved. However do these lower flows and improved fractional utilization mean increased risks or benefits? A mean fresh gas flow value of 51.4-+ 5.2 ml.kg-~.min -~ (mean --- SD) derived from ten patients does not guarantee that CO2 free inspired gas nor normal minute ventilation will be obtained when a standardized flow of 50 ml.kg -I-min -1 is used in every patient. Even though better utilization of fresh gas is achieved in this circuit, this standard deviation implies that some of these healthy patients would have been rebreathing CO2 if a FGF of only 50 ml.kg- l.min- l had been used. One must recognize that the data imply that a FGF of 50 ml.kg -~. min-t prevents rebreathing. This is not an absolute guarantee, but rather an average approximation. The rule still must be "user beware" when selecting a FGF rate for spontaneous breathing. What are the potential risks of using a convertible "universal" anaesthetic breathing system? Problems such as errors in lever placement could be avoided with experience. However, how is the "A mode" coaxial system checked for leaks? Since the reservoir bag is on the inspiratory side the Pethick test used for modified "D" systems will not apply. Manual ventilation can only be achieved in the "A mode," yet the exact fresh gas flow requirements, the amount of ventilation and other limitations have not been fully defined. Humphrey's ingenuity and progress in simplifying the more complex dual lever system should be applauded. He has eliminated several sources of error and reduced the complexity of the system. As with other systems only in time will all problems be defined. In Canada there are no minimum safe standards of either function or structure of anaesthetic breathing systems. Canadian Standards Association (C.S.A.) approval is not necessary prior to marketing new breathing systems. C.S.A. guidelines for breathing systems are to be released in the near future. The Canadian Anaesthetists' Society and its members must address this issue to ensure elimination of faults before new equipment is used in Canada as we do for new pharmaceutical products. To state that technology should be made as safe as possible helps very little. The safety of machines can almost always be improved but, at some point, the price of safety is increasing complexity and cost. A major message stressed by Humphrey et al. 1 is that anaesthetists should choose their breathing system and FGF as carefully as they choose their anaesthetic drugs. The Humphrey A.D.E. system will be useful only if the clinical anaesthetist understands the functional characteristics of both the A and D mode and the limitations are fully documented. A u r i o n s - n o u s un s y s t r m e de circuit anesthrsique universel? Dans ce numfro du Journal, Humphrey et collrgues drcrivent une version du circuit anesthrsique A.D.E.t'2 qui serait scion eux optimale pour tous les patients. Humphrey et al. ont drmontr6 que le syst~me "A.D.E. universel b. bas flow" fonctionne comme un syst~me D modifi6 (classification de Mapleson3) ou comme un circuit de Bain pour la ventilation contrrlre et lors de la mise en marche d'un interrupteur unique comme un syst~me A de Mapleson (ou circuit Lack) pour la respiration spontanre. Les donnres prrsentes sont en accord avec les 6tudes prralables utilisant le mode D. En ventilation contrrlre (partie H)2 et mode D (A.D.E.) la PCO2 artrrielle de dix patients 6tait prrvisible avec un riot de gaz frais de 70ml.kg-~.min -L et un volume courant de 10 ml'kg-1 et une frrquence respiratoire de 12 ~t 15/minute (i.e., ia ventilation contrrlre). (...truncated)